The present invention relates generally to a process for preparing ferrite single crystals by improvement of the Bridgman method.
Single crystals of ferrite such as manganese-zinc ferrite are used in narrow track magnetic recording heads which facilitate high density data storage and autodiovisual, video and digital devices. These crystals are characterized by applicability over a wide frequency range, low rubbing noise and superior mechinability and bonding properties to glass.
Vavious attempts and techniques, such as the Bridgman method, the floating zone method, have been made and employed in preparing these ferrites. Of these techniques, the Bridgman method has been proved to be most successful for growing crystals for an industrial use.
Crystals grown by the conventional methods, however, have been limited to a nominal 150 mm length by the dimensions of the furnace employed and are inhomogeneous in composition and consequently in magnetic properties.
In the ordinary Bridgman method, all of the ferrite is brought into solution before crystallization occurs. Individual components such as MnO, ZnO and Fe.sub.2 O.sub.3 might be induced to crystalize uniformly under proper temperature and atmosphere conditions. However, the ternary mixture, such as MnO, ZnO and Fe.sub.2 O.sub.3, is less controllable. As illustrated in FIG. 1, which shows schematically the binary phase relations in such a system, the liquidus and solidus curves are spread apart with the result that a melt of composition X' will crystallize as a solid of composition X and the crystal progressively increases in Mn content. Moreover, the practical length of crystals grown by the ordinary Bridgman method is limited, at most, to 150 mm primarily by the temperature gradients in the furnace. Further, the thus produced crystals of 150 mm in length has a wide variety of compositional variation along the lengthwise direction thereof, as well illustrated in FIG. 4, with the result of unproductivity and consequently of expensiveness.
An attempt has been made to overcome such disadvantages as described above, by successively charging make-up materials continuously to the initial ferrite material to the extent necessary to resupply the ingredients of the composition which decrease as the single crystal grows. However, this attempt does not improve the above-described Bridgman method but still has similar disadvantages as those of the aforementioned method. Namely, the ingredients successively suppied are those which are limited to those materials which are consumed as the crystal grows which limits the size of the crystal to the amount of the ferrite materials initially supplied to a crucible plus about 10-15 percent in volume of the initially supplied ferrite. Therefore, as described before, the practical length of crystals grown by this attempt is limited at most to 150 mm. Additionally, it is quite difficult to control when, how and what amount of ingredients or components which has decreased in the composition should be additionally supplied. Such control should be carried out experimentarily.